Wave filter



Feb. 13, 1951 w. w. MUMFORD WAVE FILTER 2 Sheets-Sheet 2 Filed June 4.1948 FIG] INSER TION LOSS 8 TAND/NG WAVE RA T10 9 a G m H a [w 0 ma 93x3mmwlwtmoz I b SLOT WIDTH IN INCHES FREQUENCY IN MEGA CILCLES INVENTOR W.W. MUMFQRD n @Ww 7% AT TORNEV Patented Feb. 13, 1 951 UNITED STATESATENT OFFICE WAVE FILTER York Application June 4, 1948, Serial No.31,137

Claims. 1

This invention relates to frequency-selective networks and moreparticularly to microwave filters for use in wave guides.

An object of the invention is to improve the transmissioncharacteristics of microwave filters. A more specific object is toequalize the insertion loss and minimize the reflection coeflicient ofsuch a filter over a wide band. A further object is to provide aresonant wave-guide element in which the sharpness of resonance, andconsequently the width of the transmitted band, may be continuouslyvaried.

In many applications it is desirable to have a microwave filter with asubstantially flat insertion loss and a reflection coetiicient which issub stantially zero over as wide a band of frequencies as possible. Afilter with such a characteristic is herein called a maximally-flatfilter. l'his type of characteristic is of particular importance, forexample, in microwave television repeaters, where the filter may belocated some distance from the receiving antenna and reflections at thefilter input will cause disturbing echoes.

In the microwave filters in accordance with the present invention theinsertion loss can be made constant and the reflection coeiiicientsubstantially zero over as wide a band as desired by using a suificientnumber of component elements. The nlter comprises a plurality ofresonant branches positioned Within a hollow wave guide with a spacingbetween branches approximately equal to an odd integral number ofquarter wavelengths at the mid-band irequency. 1n the embodiments shownthe branches are constituted by apertured transverse partitions,

or irises, which are resonant at or near the inletband frequency. Theapertures may, for example, be substantially rectangular in shape orthey may be dumbbell-shaped, or of other suitable form.

For a maximally-fiat filter in accordance with the invention, when thenumber or resonant branches exceeds two the respective band widthspassed by the branches increase progressively from the center to theends or the inter. iIllS tapering of the pass bands may be accomplishedby decreasing progressively the sharpness of resonance oi the branches.The sharpness of resonance may, for example, be controlled by changingthe size of the iris aperture, by moving the aperture off-center, or byrotating the iris in the wave guide. A feature of the invention is arotatable iris in which the sharpness of resoname may be continuouslyadjusted.

The nature of the invention will be more fully understood from thefollowing detailed description and by reference to the accompanyingdrawing, in which like reference characters are used to designatesimilar or corresponding parts and in which:

Fig. 1 is a schematic circuit of a microwave filter in accordance withthe invention comprising nine resonant branches;

Fig. 2 is a longitudinal sectional view of a wave-guide filter followingthe circuit schematic of Fig. l, in which the branches are constitutedby resonant irises;

Fig. 3 shows plan views of the irises used in the filter of Fig. 2, someof which are located off-center to decrease the band Width;

Fig. 4 shows an alternative set of dumbbelltype irises three of whichare rotated to decrease the band Width;

Fig. 5 shows filter characteristics to be referred to in thedescription;

Fig. 6 is a plan view of a rotatable dumbbelltype iris in which the bandwidth may be adjusted;

Fig. 7 is a side view of Fig. 6 as seen from the left;

Figs. 8 and 9 show curves useful in designing the filter; and

Fig. 10 is a typical insertion loss-frequency characteristic of thefilter shown in Fig. 2.

p A maximally-fiat filter is generically disclosed in United Statespatent to W. R. Bennett No. 1,849,656, issued March 15, 1932. In thatpatent it is shown that a uniform transmission characteristic over theband is obtained by the use of filter reactances which haveprogressively broader selectivities from the center of the filter towardeach end, the series reactances being the inverse of the shuntreactances and all resonant at the same frequency. It is there shownthat the reactances of the series impedances and the susceptances of theshunt impedances should be proportional to the quantity Sin 1r properlydimensioned and spaced along the guide substantially a quarterwavelength apart or an odd multiple of a quarter wavelength.

In the present application maximally-fiat wave-guide filters aredisclosed in which, instead of resonant cavities, resonant irises areemployed, properly dimensioned and spaced from one another bysubstantially a quarter wavelength or odd multiple thereof. In bothtypes of embodiments, those using cavities and those using irises, thecavities or the irises form the shunt susceptances of the filter. Theseries impedances are obtained by taking advantage of the impedancetransforming property of a quarter wavelength line. The, shunt SESCEP?tances are spaced such a critical distance from one another that everyalternate one, is transformed into a series impedance branch interposedefi'ectively between two shunt branches. In this way the effect of aladder-type filter comprising series and shunt branches, inverse to eachother and properly proportioned, is obtained. w

The schematic oi sucha filter is shown in Fig. 1 with the shunt filterbranches 20 represented as parallel-resonant circuits using inductancecoils and condensers. Ne series branches in the form of reactances arerepresented since, as explained above, these are eiiectively introducedby the use of quarter wavelength sections of connecting line, or oddmultiples thereof, between the shunt branches. As shown in connectionwith the first two shunt branches 20 at the lefthand end of the circuit,the spacing between successive branches is equal to (2ul) M4, where u isany integer and} is the wavelength in the connecting sectionof line atthe mid-band frequency f of the filter. It will be shown hereininafterhow todesign a,wave-guide filter, in accordance with the invention,which is the counterpart of the schematic shown in Fig. 1, using as theshunt filter branches 2% tuned irises, and spacing these an odd numberof quarter wavelengths apart.

Fig. 2 represents an example of such a filter constructed in accordancewith one form of the invention. It comprises a wave guide IE), whichordinarily may be a metal tube of rectangular cross-section, in whichare positioned at spaced intervals the transverse metal partitions I I,each provided with a window or aperture critically dimensioned to beresonant at the mid-band frequency f0 and provide the proper band width.There are nine partitions ll comprising four pairs h, m, p and ssymmetrically arranged with respect to the central partition 0. Theinside dimensions of the guide ware; a=1.8'72 inches and b=0.872 inch.As shown, the spacing between the partitions H is approximately equal toan odd integral number of quarter wavelengths at the mid-band frequencyIn.

It is assumed in accordance with. the, embodiment snown in Fig. 2 thatthe partitions II have apertures of the types shown in plan view in Fig.3. In each of the partitions c, h, m and p the aperture is dumbbellshaped, comprising two circular holes joined by a narrow slot. Each endpartition .9 has a narrow straight slot 24.'

These apertures have their long dimension perpendicular to the electricvector E and parallel to the a dimension, that is, parallel to the Widersides of the guide I 0.

An interesting property of the dumbbell aperture is that its resonanceis narrowed by displacing its center from the exact middle of the 4guide I!) along the a direction, that is, toward either of the narrowerside walls 25. The selectivity Qg, defined below, of such an aperture isrelated to the displacement d by where a is the width of the guide it!and Qgo is the selectivity of the centrally positioned aperture. Theresonant wavelength is but slightly afiected by these displacements.This property ermits of readil tapering the band Widths of the resonantirises as the middle of the filter is approached from each end byprogressively increasing the lateral displacement d of the dumbbellapertures as is shown in the partitions m, h and c, in Figs. 2 and 3.

It is found as a further interesting property of the dumbbell iris thatits band Width can also be varied by simply rotating it about thelongitudinal axis of the filter and this change in its angular positionalso does not materially affect its resonant wavelength. Fig. 4 showsthe par-v titions m, h and c with dumbbell irises 2B, 21 and 28progressively rotated to give the branches progressively narrower bandwidths as the center of the filter is approached. These partitions maybe substituted in the filter of Fig. 2 for those shown in Figs. 2 and 3.The selectivity of an aperture 28 whose principal axis 30 makes an angle0 with the wider or a cross-sectional dimension of the wave guide H} isgiven by where Qgo is the selectivity when 0 iszero.

As an aid to. the design of such a. filter, certain generalrelationships. will. first. begiven.

The parameter Qg, which has already been used to indicate selectivity,is called thefnormal where )\go iS the resonant wavelength in the wa veguide and A is the wavelength at one of the half power or cut-oiipoints. I V s The insertion loss function of a maximallyfiat filter forthe wavelength x is given by Lzlcq lzg gc. g0

If the bracketed term in Equation 5 is called 0,

a wavelength parameter, the expression may be written as CurveA of Fig.s showsa plot oftheinsertion loss of a maximel y fiai l r. n deciberdi'f' natesagainst thequantity 9 as abscissae. Curve B gives thestanding wave ratio. Itis noted that similar way, assigning ame totalfilter a loaded' Q defined by the cut-off wavelengths of the totalfilter as in Equation '4. This loadedQ of the total filter will becalledQT. Thus it is seen that the wavelength parameter, n, can then be given,

by combining Equations 4, and 5,

From the insertion loss curve A on Fig. 8 values of Q are obtainedcorresponding to 42 decibels and 2.4 decibels as 126 and 0.858,respectively. The ratio of these is (gyg 146.8 (8

Two values of 1'2 are found by inserting the wavelength values inEquation 6 as follows:

QFQT l2.673 1l.70z$l (9) 11.708 12.246 ll-QT lam 11.708 0'0g0 QT (10)Putting these values in Equation 8 gives from which it is determinedthat 1128.8 and hence nine filter branches are required.

Now since (S22) must be at least 126 to achieve the 42-decibel loss atf2, QT is determined by Equation 9, setting n=.8,8

(0.1586 QT)8'8=126 from which With the quantity QT and the number ofbranches thus determined, the loaded Q for each of the irises 24, 3.2,34, 33 and 23, respectively, may now be found from Equation 7 asfollows;

Q 5=10.93 sin 90=10,93

The height b of the rectangular slot 24 in each of the end partitions sfor Qgl=.1..90 can be read from the graph plotted in Fig, 9 as 0.030inch. The partitions s are made of metal and are inch thick. The width12', of the slot 24 is so chosen that the iris is resonant at thefrequency in.

It was determined experimentally that a dumbbell iris 32 of the typeshown in the partition 1; in Figs. 2 and 3, made of inch material withinch holes spaced 1 5 inch apart and connected by a slot 0.010 inch widehas a value of Q; of 5.47 and a resonant frequency of 4060 megacycles.This iris was used as the iris 32 in the partition 50 in Fig. 2.

The dumbbell irises 34, 33 and 23 all have the same aperture andthickness dimensions as the iris 32, and their displacements d from thecenter line were determined from dm=0.375 inch dh=0.447 inch dc=0A68inch The seven dumbbell irises were separated from one another bywavelength. The two end irises 24 were each separated from the nextadjacent iris 32 by A wavelength.

The measured insertion loss of the assembled filter of Fig. 2 is plottedin Fig. 10. It is seen to be quite fiat for megacycles and to have 40-decibel insertion loss at frequencies 100 megacycles away from themid-frequency on both sides. The measured value of Qg of the totalfilter is 13.7, while the design aimed at the value 10.93. The majorpart of this difference can be accounted for by the fact that in thedesign procedure outlined above no allowance was made for theselectivities of the line sections between the irises, and this wouldhave a considerable efiect where band widths as great as this areinvolved. For example, it is shown in my copending application that theconnecting line between filter elements introduces a selectivity whichis equivalent to adding a tuned circuit at each of its ends. For quarterwavelength connecting lines these circuits each have a Qg of 1r/8 andfor wavelength line sections each of these circuits has a Qg of 31r/8,or 1.18. This can be compensated by subtracting an equivalent amountfrom the Qg of the filter branches to which the lines are connected. Ifthis is done, the center iris will have its Qg reduced by 2 1.18 or 2.36to compensate the selectivity inherent in the two line sectionsconnected to it and the observed QT of 13.7 would then be reduced to11.34 which is a close approach to the design objective, 10.93.

If, instead of obtaining the increased selectivi-i ties of the fivemiddle irises 34, 33 and 23 in the above example by moving them offcenter as shown in Figs. 2 and 3, their selectivities are increased byrotating the irises in the wave guide as shown in Fig. 4, the requiredangle of rotation 0 for the irises Z3, 2? and 28 is determined by therelation In stru t n a filter with progrcssivem tated dumbbell irises inwhich each end iris is m a t all the others except that t s not e tated,the angle of rotation 0 for the other irises is determined by therelation sin 9 cos" tihSl.f1fiaIn,d-3. as. similar expressionfor the;oft- '-center-;:case;;is:obtained: byvsettin .inrEquation 13. V

:Whilethe illustrative examples that have. been .givenemploy ninefilterbranches it is found that,

using the dumbbell iris,-very good filter-characteristics can beobtained by use of as few as two irises. For example, a filter using twoon-center dumbbell irises-dimensioned'es above and each having aQg:'.4:7,iafid spaced a quarter wavelength apart, displayed theexcellent properties shown by the (measured) solid line graph of Fig. 5,where the standing Wave ratio .in-decibels is plotted against frequency.superposed on this same plot is a dotted line characteristic of;-atwobranch =filterusingresonant cavities (of slightly .higher'Q 'Thetwo-iris filter characteristic is free vof .the double-humped effectshownin the two-cavity filter characteristic.

Rotatable irises have the attractive feature of permittingofadjustmentby external control so that .an assembled filter .can beadjusted and measured until the best characteristic is obtained. One wayin which an adiustably rotatable iris can 'be provided in a wave guidein accor-dance. with this invention is illustrated in'Figs. 6 and '7.

The guide tube l 0 ismade in sections each provided withalfiange l 5 atthe end where it is to be joined to the next section. The dumbbell orother'shaped aperture .35 is made in ametal disc A 5 [containing arcuateslots l2, 52 at such radial distance from the longitudinal center'lineof the guideas toallowthe clamping screws "i3, h; to pass through theseslots for all angular positions of disc H covering a quarter of a fullrotation, thus allowing any required angular adjustment of the dumbbellaperture 36 to be made.

'The complete filteris assembled with a disc H included at eachj'oint'for as many sectionsas desired. The approximate angular positionof each disc H is set as the filter is assembled,

'either'from predetermined values obtained by calculation ormeasurement, or by measuring'the Q during the assembly process. Theassembled 'filter .is clamped at all 'joints and measured.'Subsequent'adjustments can then be made at any time 'by' loosening thescrews'i3, It at any or all joints, changing the angular setting of thedisc or discs H, and again tightening the screws.

The invention is not to be construed as limited to the details ormagnitudes disclosed. herein for illustration, since the scope of theinvention is defined by the claims.

What is claimed is:

1.A wave filter for transmitting a band of electromagnetic wavescomprising a wave guide and a plurality of transverse partitions thereinhaving a spacing approximately equal to an odd integral number ofquarter wavelengths at the mid-band frequency, each of said partitionshav- -ing therei11- .an-aperture dimensioned to constitute an irisresonantat said frequency, and. two of said irises being ofsubstantially the samesize and shape but difierentlypositioned withrespect to the longitudinal axis 0f:- said guide.

' 8 ii -isAzfi l fi hi escord ee r rithe laisn. zierwhifi the Centers Mqisesaralo atefl t-s -fi ent distances from said axis.

,3. A filter in accordancewith claim 1 in which corresponding axes ofsaid two irises make differ- 4. LA: filter int accordancewith cla m Iwl; c one of said partitions isrotatabl -abou i s: 3 ter.

)5. A filter in accordance with ,claim 1 comprising more than two ofsaid irises, the selectivities of'said irises beingsubstantiallyproportional. to the quantity where re denotes the ;order of-,fthe=.iris counting trom the nearer-endiand nis, theltqtal-numben ofirises.

6. A filter in accordance with claim 1 comprising. more. than two :ofsaid irise.S..1 ?he. -,centers of said irises being :;locatedprogressiyely iarther from said axis as the center of the filter isapproached from either end.

7. A filter in accordance with claim 1 comprising more than two of saidirises, the principal axes of said irises making progressively smallerangles with the electric vector of said waves as the-center of thefilter-is approached irem either end.

8. A wavefilter .for transmitting -..-a bandcof electromagnetic wavescomprising. a -;,wav,e; guide and at least three transverse partitionstherein having a spacing approximately equal to an odd integral numberof quarterwavelengths at the mid-band irequency, each of said partitions:having therein an elongated aperture is dimensioned to be resonant at;approximately said frequ ncy, and said apertutes 'rbecomingprogressively more asymmetric with respect to a center line of thecross-section of aid guide as the center of-thefilteriisgapproachedgfromeither end.

9. A filterv in accordance with claim 8 in which the centers of saidapertures are spaced progressively farther from thelongitudinalaxis ofsaid guide as the center ofthe filter-41s approached .fromeither end.

10. A filter inaccordance with claim 81in which the major axes of said-a-pertures,,m ake pro res- -,sively decreasing angles with the electricvector of said waves as the center of the filter is approached fromeither end.

WILIJAM W. MUMFORD.

SiD.

REFERENGES CITED 'I -heiollow-ing references areot recordsini'thefile-of this patent:

" NI ED STATES PAT TS OTHER REFERENCES fllllicrowave Filters UsingQuarter=Wave gCouplings, by Fano and Lawson, 15R. Proceedings, vol.j 35,N0. 11,. Nov. 1947.

